TUCSON, Ariz. — Research led by scientists from the University of Arizona and the National Institute for Amazon Research in Brazil has discovered the reason for a discrepancy between large seasonal changes in photosynthesis in the Amazon forest.

Most tropical forests appear green and photosynthesize all year long, but scientists long have debated whether — and by how much — the "greenness" of these forests can increase and decrease seasonally. In particular, scientists were puzzled by the seeming discrepancy between large seasonal changes in photosynthesis seen from towers on the ground versus smaller changes seen from satellites in space.

An international team of scientists from the U.S., Brazil and Australia used high-tech forest photography to discover the reason for the discrepancy: Synchronization of new leaf growth with old leaf loss causes the large seasonal increases in photosynthesis and greenness measured from forest towers and is also consistent with the observations from space.

The findings, published Feb. 26 in the journal Science, should help scientists improve their predictions of how these forests respond to future climate change by replacing assumptions about how tree canopies can increase or decrease their level of photosynthesis.

Photosynthesis is how all plants live, using sunlight to manufacture sugars and carbohydrates (their "food" supply) from atmospheric carbon dioxide, giving oxygen back to the atmosphere along the way.

Scientific models that predict how climate and vegetation interact with each other long have represented tropical forest trees in a simplistic way, assuming they have consistent canopy greenness throughout the year — unlike the dramatic cyclical changes in temperate forests, heralded by vibrant reds and yellows.

"Across the landscape, at large scales seen by satellites, the forests always look evergreen," said Jin Wu, a postdoctoral research associate at the U.S. Department of Energy's Brookhaven National Laboratory. "However, when we used cameras to look carefully every day at the trees one by one, it was very exciting. We saw dramatic leaf loss and rapid growth spurts of new leaves that couldn't be easily seen by satellites."

Wu is the lead author on a study completed while he was a Ph.D. student with senior author Scott Saleska, associate professor of ecology and evolutionary biology at the UA.

The discovery through photos showing that synchronized old leaf death and new leaf growth happened at the same time as the annual seasonal increase in forest photosynthesis was the key to the whole story, explained Saleska, who compared photosynthesis to human metabolism.

"Simply put, when it comes to leaves and photosynthesis, it is no different than with us humans: Age matters," Saleska said. "When you swap out a bunch of old leaves and exchange them for young new ones instead, overall photosynthesis has to go up, even if the total amount of leaves doesn't change very much."

The amount of leaves being exchanged is surprisingly high. When collaborating authors Bruce Nelson and Aline Lopes, staff researcher and graduate student, respectively, at Brazil's National Institute for Amazon Research, first started looking at the camera images of the forest at two sites near Manaus, they were astonished.

"Fully a third of the trees in this forest lose most or all of their leaves, and then within just a month grow them all back again," Lopes said. "It's an amazing example of how dynamic individual trees can be, even in a forest that overall is classified simply as 'evergreen.'"

Added Saleska: "There is the old saying that you can't see the forest for the trees. What this work shows is that sometimes, the opposite is true: You can't see the forest until you also see the trees — and the individual leaves, too."

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The research was supported by the National Science Foundation's Partnership for International Research and Education, the National Aeronautics and Space Administration, the University of Arizona's Agnese Nelms Haury Foundation, the GoAmazon project (funded jointly by the U.S. Department of Energy and the Brazilian state science foundations in São Paulo state and Amazonas state), the Brazilian Ministry of Science Technology, the Max Planck Society and the German Federal Ministry of Education and Research.